John Abrahamson

The surface energies of graphite

Abstract A discussion is presented describing the possible influence of the internal surface energy of polycrystalline and pyrolytic graphites on three phenomena: (a) the thermodynamic “standard” state of carbon, (b) the graphitization process, and (c) the structural stability of graphites at high temperatures. In order to do this, the surface enthalpies and free energies of the basal surface and edge faces have been calculated from a number of sources, and their temperature dependence up to 3000K discussed. Suggestions have been made for methods to measure both basal surface and “edge” surface energies more directly.

Ball lightning caused by oxidation of nanoparticle networks from normal lightning strikes on soil

Observations of ball lightning have been reported for centuries, but the origin of this phenomenon remains an enigma. The ‘average’ ball lightning appears as a sphere with a diameter of 300 mm, a lifetime of about 10 s, and a luminosity similar to a 100-W lamp. It floats freely in the air, and ends either in an explosion, or by simply fading from view. It almost invariably occurs during stormy weather. Several energy sources have been proposed to explain the light, but none of these models has succeeded in explaining all of the observed characteristics. Here we report a model that potentially accounts for all of those properties, and which has some experimental support. When normal lightning strikes soil, chemical energy is stored in nanoparticles of Si, SiO or SiC, which are ejected into the air as a filamentary network. As the particles are slowly oxidized in air, the stored energy is released as heat and light. We investigated this basic process by exposing soil samples to a lightning-like discharge, which produced chain aggregates of nanoparticles: these particles oxidize at a rate appropriate for explaining the lifetime of ball lightning.

Carbon fibre layers on arc electrodes—I: Their properties and cool-down behaviour

Abstract A thick mat of fine fibres and crystallites has been found on the surfaces of graphite and carbon anodes after low current arc operation in nitrogen. Similar operation in air leaves no trace of the fibres. The dimensions and other properties have been studied by scanning and transmission electron microscopy. Radiance decays observed at the anode face of the “standard carbon arc” after rapid current cut-off have been studied over a range of wavelengths. The temperature drop at the surface is too rapid for bulk graphite cooling, as has been pointed out by Lozier and Null. Purely radiative and conductive-radiative cooling mechanisms in surface layers have been considered, and it is found that conduction through the fibre layer and a porous layer beneath the fibres controls the cooling rate for the first 100 μs.

Graphite sublimation temperatures, carbon arcs and crystallite erosion

Abstract Past efforts to measure the sublimation temperature of graphite by electric arc and cavity experiments are reviewed. Critical difficulties of interpretation of various anode reflectance measurements are pointed out, and resolved in terms of the emission of small graphite crystallites from graphite, and the existence of a shallow porous layer on the anode. It is argued that crystallites, which may form the major part of the mass loss, can partly obscure the surface from view. Also in the case of the “standard” carbon anode it is reasoned that the temperature of the observed surface can be controlled by thermal conduction to finely divided subliming carbon, which carries the positive charge of the quiet carbon arc close to the anode. It was concluded that the sublimation temperature of graphite at one atmosphere pressure lies between 3895 and 4020 K, and that little further experimental work needs to be done to define it more precisely, and to establish a new high temperature standard.

Theoretical studies of interstitials in graphite

Abstract CNDO (Complete neglect of differential overlap) calculations are reported for a model of interstitial C and C2 species between graphite planes, modelled by two C16H10 graphite like molecules. For a single C atom interstitial the optimum position is about 0.82 A above the centre of a ring. The interstitial is bonded to one plane with only a weak interaction with the plane above. For the C2 interstitial, the optimum position is with each C atom 0.83 A above the ring in adjacent layers. Agreement with experimental values for the migration energy of an interstitial is not found, and may be achieved if deformation of the graphite layers is also taken into account.

Permanent electric dipoles on gas-suspended particles and the production of filamentary aggregates

The shape of an aggregate of particles is the result of the forces acting in its formation, and in turn strongly influences the properties of the aggregate. In this paper we investigate the forces acting on particles in the gas phase to form extended chains. Electrical charge on the particles is normally invoked to explain these chains, in the form of charges of opposite sign on different particles, and dipoles induced by imposed electric fields. We emphasize another possibility; permanent electric dipoles composed of separated persistent charges of opposite sign on the same particle. We discuss (a) contact charging for micron sized dielectric particles and (b) chemical charging during growth of oxide layers on metal nanoparticles, noting experimental evidence supporting the existence of dipoles. A mapping of different aggregate forms from dipolar particles has been attempted, including magnetic and electrical dipoles, taking account of concentration, dipole strength and time, all in dimensionless form. Turbulent diffusion is found important with flow situations. Aggregation of dipole particle chains into ball-like objects is mentioned with respect to ball lightning.

Influence of entry duct bends on the performance of return-flow cyclone dust collectors

Abstract Major changes in cyclone collection efficiency are found with different upstream bends in the feed pipe, under otherwise the same cyclone design and conditions. These effects are shown for a number of plant-sized cyclones, and for a laboratory-scale cyclone, for bends up-flow to horizontal, down-flow to horizontal, and bending towards the barrel and away from it. It is expected that this behaviour occurs wherever solids inertia is important, such as with strong large aggregate formation (the example studied here was wood pulp) or with high solids loading (milled coal suspension). The efficiency changes are explained by stronger or weaker radial penetration of the cyclone vortex by the incoming coherent solids stream. This same idea, after allowing for the properties of strand conveying in the inlet duct, could also give a new reason for the generally observed increase in efficiency with solids loading, and other cyclone behaviour.

Parameters affecting deposition of multiwalled carbon nanotubes on a continuously fed substrate using arc discharge

Carbon nanotubes have been extensively studied since their discovery (Iijima, 1991) due to their valuable structural and electronic properties. Direct growth of nanotubes on substrates is essential for their integration into various applications, as individual manipulation into position is both difficult and expensive due to their size. A unique single-step method has been developed of continuously depositing nanotubes on a carbon substrate using an arc discharge at atmosphere pressure (Abrahamson, 2005). This method differs from the conventional arc discharge method in that the nanotubes are grown at low currents on a moving substrate surface which acts as one of the electrodes. The effects of inter-electrode gap, buffer gas flow through the porous substrate and substrate speed on the yield and morphology of carbon nanotubes are investigated. It was found that an inter-electrode gap range of 2.5 - 6.0mm is optimal for nanotube occurrence on the carbon substrate. Providing a flushing gas flow into the arc through the substrate reduced the mean diameter but not the number of nanotubes and markedly reduced the number of attached nanoparticles. The residence time of the substrate in the arc was varied by changing substrate velocity, and this was found to be critical for nanotube formation. From the parameters explored, it appears that the substrate temperature alone governs nanotube formation and this occurs at temperatures lower than reasonable sublimation temperatures. This latter fact together with the lack of influence of gas flush, indicate that carbon vapour is unlikely to dominate the formation of nanotubes in arcs.

Ball lightning comes down to Earth

Ball lightning – a slow-moving ball of light about 30 centimetres across that is occasionally seen at ground level during thunderstorms – has puzzled scientists for centuries. Benjamin Franklin, for instance, made several attempts to observe this mysterious phenomenon over 200 years ago. In one attempt he tried to make ball lightning from ordinary lightning – inspired by reports of a Russian contemporary who had been killed by such a ball after directing a lightning strike into his laboratory.

Influence of anode surface temperature in a continuously-fed arc discharge depositing carbon nanotubes

Mass production of carbon nanotubes (CNTs) by a cost effective process is still a challenge for further research and application of CNTs. The group has focussed on the deposition of CNTs on a continuously-fed carbon substrate via arc discharge at atmospheric pressure. This process produces MWNTs using carbon from the substrate. The method differs in other respects from the conventional batch arc discharge method by using lower currents (< 20 A) and larger inter-electrode gaps. To help define the local conditions of nanotube growth, the substrate surface temperature (Ts) was measured by optical pyrometry. Here, it was reported the influence of inter-electrode gap, substrate velocity and arc current on this temperature. It was found that carbon nanotube growth is favourable over a certain temperature range and retention time in the arc. To further understand the effect of arc parameters, a computer simulation to model the arc plasma was used. Computational fluid dynamic (CFD) software, Comsol Multiphysics, was used to simulate the temperature distribution and flow properties of the arc plasma. It was found necessary to include dusty plasma conductivity near to the electrodes to adequately represent observed arc behaviour.